The two available crystallographic structures of cobalamin dependent enzymes, the 27 kDa fragment of the methylcobalamin-dependent enzyme, methionine synthase, from Escherichia coli [C. L. Drennan et al. Science, 266, 1669 (1994)] and the 5[unreadable]-deoxyadenosylcobalamin-dependent enzyme methylmalonyl-coenzyme A mutase from Propionibacterium shermanii [F. Mancia et al. Structure, 4, 339 (1996)], show striking similarities despite the differences in reaction mechanism. In particular, the 5,6-dimethylbenzimidazole group is detached and replaced by a histidine group of the enzyme. We have analyzed Extended X-ray Absorption Fine Structure (EXAFS) spectroscopic data for both 5[unreadable]-deoxyadenosylcobalamin and aquocobalamin bound to methylmalonyl-coenzyme A mutase in the absence of substrate. The analysis is conducted with a suite of programs called AUTOFIT 1.0 [Chance et al., Biochemistry, 1996, 35, 9014], which allows an evenhanded comparison of the goodness-of-fit of the EXAFS data to a varied grid of simulations based on the ab initio EXAFS code FEFF 6.01. The x-ray edge data indicate an increase in effective nuclear charge of the metal ion of the enzyme bound 5[unreadable]-deoxyadeonsylcobalamin compared to the corresponding free cobalamin and the EXAFS results show small decreases in equatorial and no significant change in the Co-C bond length.